Cox Proportional Hazards: Add stratification support

Pivotal Tracker: 55945460

Other authors:
    Feng, Xixuan (Aaron) <xfeng@gopivotal.com>
    Shengwen Yang <syang@gopivotal.com>
    Rahul Iyer <riyer@gopivotal.com>

All Changes:
    - Updated Cox interface and changed function name to coxph
    - Added strata and optimizer_params parameters
    - Moved all help messaging to Python
    - Optimized the overall process (remove the intermediate step)
    - Replace plpgsql with sql for the overloaded functions
    - Implement stratified cox
    - Added Hessian to Cox Output Table
    - Added <output_table>_summary containing model details and its usage message
Cox Proportional Hazards: Add function for PH assumption test

Pivotal Tracker: 55945460

Other authors:
    Shengwen Yang <syang@gopivotal.com>
    Qian, Hai <hqian@gopivotal.com>
    Feng, Xixuan (Aaron) <xfeng@gopivotal.com>

All changes:
    - Added aggregate to compute Schoenfeld residual, element-wise correlation,
    and chi-square/p-values
    - Merged the design doc for stratified Cox including the residual
    computation part and chi-square computation
    - Changed function names
        coxph --> coxph_train
        zph --> cox_zph
Documentation: Improve formating for various modules

Changes:
    - Reformat doc in Early Stage Development modules to new template.
    - Treat online help consistently
        1. Remove from contents
        2. Remove online help section
        3. Make online help as the first step in examples
    - Remove group from SVDMF to eliminate it from docs
    - Change incorrect title in SVD documentation